Over the past decade, it has become clear that viruses have evolved strategies to evade attack mediated by antibody, complement (C'), or the cellular immune system. Holpos simplex virus (HSV) encodes glycoproteins that modify the ability of antibody and C' to clear virus. It is our hypothesis that these glycoproteins account for the observation that humoral immunity plays a limited role in defense against HSV, since the virus is capable of reducing the effectiveness of antibody and C' attack. Our research focuses on the role of HSV glycoprotein C (9C), which binds C' component C3b, a pivotal protein in the C' cascade. 9C also blocks binding of properdin (P) and C' component C5 to C3b. These actions accelerate the decay of an essential enzyme in the alterative C' pathway and decrease the ability of both classical and alternative C' pathways to mediate terminal lytic events. We studies the role of HSV-1 9C in virulence using a mucosal route of infarction by comparing wild type (wt), 9C mutant and rescued viruses. We demonstrated the importance of 9C in virulence based on: i) 9C mutants cause lease severe disease;' and ii) vaginal titers of 9C mutants are significantly lower than wt or rescued virus. To demonstrate a role for C' in HSV-1 infection we used cobra venom factor to deplete C', and we studied virulence in a unique colony of C3 deficient guinea pigs. C'depletion or C3 deficiency leads to more severe disease and increased viral titers. Three laboratories with differing expertise have combined their efforts to examine the role of HSV 9c in immune evasion. This grant focuses initially on HSV 9c-1 to define the biologic relevance of 9c-c' interactions. We will then apply this knowledge ot determine if HSV 9c-2 functions similarly. In Aim 1, recombinant viruses that are deficient in C3 binding domains on 9c but intact for other 9c functions, such as attachment and infection of polarized cells, will be studied in C' deficient and C' intact animals to further evaluate 9c as a virulence factor. Aim 2 examines 9c structure as it relates to C' binding or virus attachment, information that is required for preparing a panel of mutants for in vivo studies. Aim 3 attempts to further our understanding of mechanisms by which 9c modulates C' activation.